12.) 9/10 divided by 3/4

15.) 5/6 divided by 3/10

17.) 5/8 divided by 3/5

18.) 21/32 divided by 7/8

When you divide fractions you multiply by the reciprocal.
12.) 9/10 times 4/3 is 36/30. This simplifies to 18/15.
15.) 5/6 times 10/3 is 50/18. Simplifies to 25/9.
17.) 5/8 times 5/3 is 25/24.
18.) 21/32 times 8/7 is 168/224. Simplifies to 21/28.